The present invention relates to an evaporation device for drying sludges, the device including a heatable, hollow cylindrical evaporator that is oriented approximately horizontally and is provided at one end with a product inlet and at the other end with a product outlet. In the interior of the evaporator where it is charged with the product there is disposed an externally driven rotor which is equipped with essentially radially oriented vanes that extend essentially over the length of the evaporator.
Such an evaporation device has been used for many years to dry wet products, particularly sludges from waste water systems. The vanes of the internally rotating rotor which extend essentially over the length of the evaporator and whose edges end at a small distance from the interior wall of the evaporator cause the sludges to be deposited on the heated interior wall of the evaporator in the form of a thin product film, with the rotor vanes continuously transporting the product through the evaporator and discharging it at the product outlet in the form of an at least partially dry granulate.
In this connection, it has been found to be necessary to make the interior surface of the evaporator smooth and cylindrical throughout in order to obtain a high heat transfer rate to the product or sludge film. However, with many such sludges there exists the problem during drying that, with decreasing water content, the material to be dried, which is introduced at the product inlet as a pumpable product, goes through a viscous intermediate phase which leads to the formation of large lumps. In certain regions of the "viscous" phase of the sludge to be dried, there will initially be product baking onto the vane edges which prevents the formation of a product film because the interior surface is "wiped" in this region and the drying process is interfered with. From time to time the baked product comes loose in coarse agglomerates. Although these coarse agglomerates are substantially broken up again by the rapidly rotating rotor vanes, they result in a large percentage of fine grained material or dust. Moreover, the heat transfer to a product to be dried, which is present in the form of coarse lumps, is considerably reduced by their small contact surface toward the heated interior wall. The resulting coarse lumps have such a configuration that they have a relatively thin, dry outer surface which encloses a still relatively wet product quantity. If such lumps reach the product outlet, their breaking open may cause sticking, baked-on product, etc. in subsequently connected devices.